refactor: improve debug logging

[stepankuzmin]

Closes #83

[codecov]

Codecov Report

Merging #264 (bc7369d) into master (206307f) will increase coverage by 0.03%.
The diff coverage is 47.82%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   49.51%   49.54%   +0.03%     
==========================================
  Files          14       14              
  Lines        1735     1748      +13     
==========================================
+ Hits          859      866       +7     
- Misses        876      882       +6     

Impacted Files	Coverage Δ
src/bin/main.rs	0.80% <0.00%> (ø)
src/db.rs	50.00% <38.46%> (-3.66%)	⬇️
src/server.rs	39.03% <41.66%> (+0.55%)	⬆️
src/table_source.rs	66.34% <50.00%> (-1.31%)	⬇️
src/composite_source.rs	88.67% <100.00%> (ø)
src/config.rs	55.20% <100.00%> (ø)
src/function_source.rs	88.67% <100.00%> (+0.67%)	⬆️
src/utils.rs	83.33% <100.00%> (ø)

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